1. Field of the Invention
The present invention relates to a method and apparatus for measuring a dimension of a pattern formed on a photo-mask.
2. Description of the Related Art
In general, an optical microscope is used as a device for measuring a dimension of a pattern formed on a photo-mask, because it can prevent the photo-mask from being damaged and it can provide good measurement repeatability when the same position is repeatedly measured. According to a conventional dimension measurement method using an optical microscope, an image of a pattern is first obtained by the optical microscope. Then, a certain relative value relative to the maximum value and minimum value of the image intensity profile (for example, the medium value between the two values) is set as a threshold, and the distance between positions reaching the threshold is output as a measured value of the dimension of this portion (for example, see Jpn. Pat. Appln. KOKAI Publication No. 9-196624 (Patent Document 1)).
However, where a pattern dimension is too small for an optical microscope to sufficiently resolve, the position coordinate identified by a threshold, determined in accordance with a relative value as described above, does not agree with a pattern edge. As a result, a measured value of a dimension disadvantageously greatly differs from the actual value of the dimension.
As a technique for solving this problem, there is proposed a method to be performed as follows (for example, see Photo-mask CD metrology at the 100 nm node”, John Allsop, et. al., Proceeding of SPIE, Vol. 4754, pp. 745-757 (2002) (Non-Patent Document 1)). Specifically, dimensions of a plurality of patterns having different dimensions are measured by an optical microscope. Further, actual values of the dimensions are obtained by a scanning electron microscope having a resolving power higher than that of the optical microscope. The measured values of the dimensions obtained by the optical microscope and the actual values of the dimensions are plotted to form a curved line showing the relationship between them. Then, this curved line is used to convert a measured value of a dimension to an actual value of the dimension.
However, there is a case where a space pattern (clear pattern) is interposed between line patterns (opaque pattern), and at least one of the line patterns cannot be sufficiently resolved by a microscope. In this case, even where a curved line showing the relationship between measured values of dimensions and actual values of the dimensions is used to convert a measured value of a dimension, the value thus obtained by conversion does not necessarily agree with the actual value of the dimension.
As described above, conventionally, where a dimension of a pattern formed on a photo-mask is measured by an optical microscope, if the resolving power is not enough for the measurement target pattern or adjacent pattern, a value obtained from a measured value of a dimension does not necessarily agree with the actual value of the dimension. Accordingly, it is preferable to provide a pattern dimension measurement method and dimension measurement apparatus using a microscope, which allow a dimension of a photo-mask pattern to be measured with high accuracy, even if the resolving power is not enough for the measurement target pattern or adjacent pattern.